Vanilla raids mostly suck
UBRS/LBRS: not really a raid
ZG: Trolls so already wack. Also very easy.
BWL: Actually interesting mechanics, good story, but farming it for Elementium forever means I can't stand to even think about it
MC: legit the worst raid of all time. Patrolling bosses, long as shit, annoying attunement, ugly af
Ony: One boss, fun but kinda meh for loot. Neat story stuff for Alliance though.
AQ: Fun bosses, you get a bug mount, overall both 20 and 40 are pretty good
Naxx: If you liked it in LK you'll like it in Vanilla. Pretty much the same but much more punishing. Best raid until Uld imo.